Impulse Control Disorders in Parkinson's Disease: An Overview of Risk Factors, Pathogenesis and Pharmacological Management

Impulse control disorders in Parkinson's disease are relatively common drug-induced addictive behaviours that are usually triggered by the dopamine agonists pramipexole, ropinirole and rotigotine. This narrative review aimed to provide a comprehensive overview of the current knowledge of impulse control disorders in Parkinson's disease. We summarised the prevalence, clinical features, risk factors and potential underlying mechanisms of impulse control disorders in Parkinson's disease. Moreover, recent advances in behavioural and imaging characteristics and management strategies are discussed. Early detection as well as a tailored multidisciplinary approach, which typically includes careful adjustment of the dopaminergic therapy and the treatment of associated neuropsychiatric symptoms, are necessary. In some cases, a continuous delivery of levodopa via a pump or the dopamine D1 receptor agonist, apomorphine, can be considered. In selected patients without cognitive or speech impairment, deep brain stimulation of the subthalamic nucleus can also improve addictions. Finding the right balance of tapering dopaminergic dose (usually dopamine agonists) without worsening motor symptoms is essential for a beneficial long-term outcome.

Detecting Subtle Cognitive Impairment in Patients with Parkinson's Disease and Normal Cognition: A Novel Cognitive Control Challenge Task (C3T)

Patients with Parkinson's disease (PD) often show early deficits in cognitive control, with primary difficulties in flexibility and relatively intact stable representations. The aim of our study was to assess executive function using an ecologically valid approach that combines measures of stability and flexibility. Fourteen patients without cognitive deficits and sixteen comparable control subjects completed a standardized neuropsychological test battery and a newly developed cognitive control challenge task (C3T). We found that the accuracy of C3T performance decreased with age in healthy participants and remained impaired in PD patients regardless of age. In addition, PD patients showed significantly lower overall performance for cognitive control tasks than healthy controls, even when they scored in the normal range on standardized neuropsychological tests. PD Patients responded significantly faster than healthy control subjects regarding flexible cognitive control tasks due to their impulsivity. Correlations showed that the C3T task targets multiple cognitive systems, including working memory, inhibition, and task switching, providing a reliable measure of complex cognitive control. C3T could be a valuable tool for characterizing cognitive deficits associated with PD and appears to be a more sensitive measure than standardized neuropsychological tests. A different assessment approach could potentially detect early signs of the disease and identify opportunities for early intervention with neuroprotective therapies.

The contribution of executive control dysfunction to freezing of gait in Parkinson's disease

OBJECTIVE

An executive dysfunction is supposed to contribute to freezing of gait (FoG) in Parkinson's disease. We aimed to investigate at a behavioral and cortical levels whether an attentional load (particularly, a conflicting situation) can specifically impact preparation and execution phases of step initiation in parkinsonian patients with FoG.

METHODS

Fifteen patients with FoG, 16 without and 15 controls performed an adapted version of the Attention Network Test, with step initiation as response instead of the standard manual keypress. Kinetic and kinematic features of gait initiation as well as high-resolution electroencephalography were recorded during the task.

RESULTS

Patients with FoG presented an impaired executive control. Step execution time was longer in parkinsonian patients. However, the executive control effect on step execution time was not different between all groups. Compared to patients, controls showed a shorter step initiation-locked alpha desynchronization, and an earlier, more intense and shorter beta desynchronization over the sensorimotor cortex. Even though controls were faster, the induced alpha and beta activity associated with the effect of executive control didn't differ between patients and controls.

CONCLUSIONS

Tasks of conflict resolution lead to a comparable alteration of step initiation and its underlying brain activity in all groups. Links between executive control, gait initiation and FoG seem more complex than expected.

SIGNIFICANCE

This study questions the cognitive hypothesis in the pathophysiology of freezing of gait. Executive dysfunction is associated with FoG but is not the main causal mechanism since the interaction between attention and motor preparation didn't provoke FoG.

Prolonged Processing Time for Manual and Vocal Responses in Parkinson Disease: A Psychological Refractory Period Paradigm Study

PURPOSE

The purpose of this study was to examine a potential increased cognitive processing bottleneck within Parkinson disease (PD) by extending a previous overlapping task methodology. Additionally, this study extends previous overlapping task methodology in PD to examine the influence of modality (vocal vs. manual) on response delays in overlapping tasks in PD.

METHOD

This study used the psychological refractory period (PRP) paradigm (overlapping-task paradigm) to study processing limitations as participants complete two tasks that increasingly overlap in time. Three levels of temporal overlap of tasks were utilized to vary cognitive demands on manual and vocal response time tasks. Ten participants with PD (PwPD) and 12 participants without PD were included in this study.

RESULTS

Participants with PD demonstrated response time delays across temporal overlap conditions (likely indicating motor deficits) along with a larger increase in response delays in the most overlapped, cognitively taxing condition (likely indicating longer central processing bottleneck). Additionally, modality did not influence response times differently in overlapping task conditions or within participant groups.

CONCLUSION

An extension of previous overlapping task methodologies within a complex task was successful in demonstrating an increased central processing deficit across manual and vocal response delays in PD, regardless of modality of response.

Cognitive control in Parkinson's disease

Cognitive control is the ability to act according to plan. Problems with cognitive control are a primary symptom and a major decrement of quality of life in Parkinson's disease (PD). Individuals with PD have problems with seemingly different controlled processes (e.g., task switching, impulsivity, gait disturbance, apathetic motivation). We review how these varied processes all rely upon disease-related alteration of common neural substrates, particularly due to dopaminergic imbalance. A comprehensive understanding of the neural systems underlying cognitive control will hopefully lead to more concise and reliable explanations of distributed deficits. However, high levels of clinical heterogeneity and medication-invariant control deficiencies suggest the need for increasingly detailed elaboration of the neural systems underlying control in PD.

The LC-NE system as a potential target for neuromodulation to ameliorate non-motor symptoms in Parkinson's disease

Parkinson's disease (PD) is associated with severe motor symptoms but also with several non-motor symptoms (NMS). A substantial reduction of norepinephrine (NE) levels in various brain regions reflecting an extensive loss of innervation from the LC has been assumed as causal for the development of NMS and specifically of attentional impairments in PD. Transcutaneous auricular vagus nerve stimulation (taVNS) is a new, non-invasive neurostimulation method supposed to modulate the LC-NE system in humans. In the current opinion paper, we introduce taVNS as a systemic approach to directly affect NE neurotransmission in healthy as well as clinical populations and discuss its potential as therapeutic option for the treatment of NMS, specifically attentional deficits, in patients with PD. Here, we first describe the LC-NE system and discuss how LC-NE dysfunction might affects cognition in PD before detailing the mode of action of taVNS and proposing its use to modulate cognitive deficits in these patients.

Processing of Degraded Speech in Brain Disorders

The speech we hear every day is typically "degraded" by competing sounds and the idiosyncratic vocal characteristics of individual speakers. While the comprehension of "degraded" speech is normally automatic, it depends on dynamic and adaptive processing across distributed neural networks. This presents the brain with an immense computational challenge, making degraded speech processing vulnerable to a range of brain disorders. Therefore, it is likely to be a sensitive marker of neural circuit dysfunction and an index of retained neural plasticity. Considering experimental methods for studying degraded speech and factors that affect its processing in healthy individuals, we review the evidence for altered degraded speech processing in major neurodegenerative diseases, traumatic brain injury and stroke. We develop a predictive coding framework for understanding deficits of degraded speech processing in these disorders, focussing on the "language-led dementias"-the primary progressive aphasias. We conclude by considering prospects for using degraded speech as a probe of language network pathophysiology, a diagnostic tool and a target for therapeutic intervention.

Make a Left Turn: Cortico-Striatal Circuitry Mediating the Attentional Control of Complex Movements

BACKGROUND

In movement disorders such as Parkinson's disease (PD), cholinergic signaling is disrupted by the loss of basal forebrain cholinergic neurons, as well as aberrant activity in striatal cholinergic interneurons (ChIs). Several lines of evidence suggest that gait imbalance, a key disabling symptom of PD, may be driven by alterations in high-level frontal cortical and cortico-striatal processing more typically associated with cognitive dysfunction.

METHODS

Here we describe the corticostriatal circuitry that mediates the cognitive-motor interactions underlying such complex movement control. The ability to navigate dynamic, obstacle-rich environments requires the continuous integration of information about the environment with movement selection and sequencing. The cortical-attentional processing of extero- and interoceptive cues requires modulation by cholinergic activity to guide striatal movement control. Cue-derived information is "transferred" to striatal circuitry primarily via fronto-striatal glutamatergic projections.

RESULT

Evidence from parkinsonian fallers and from a rodent model reproducing the dual cholinergic-dopaminergic losses observed in these patients supports the main hypotheses derived from this neuronal circuitry-guided conceptualization of parkinsonian falls. Furthermore, in the striatum, ChIs constitute a particularly critical node for the integration of cortical with midbrain dopaminergic afferents and thus for cues to control movements.

CONCLUSION

Procholinergic treatments that enhance or rescue cortical and striatal mechanisms may improve complex movement control in parkinsonian fallers and perhaps also in older persons suffering from gait disorders and a propensity for falls. © 2021 International Parkinson and Movement Disorder Society.

Reduction of falls in a rat model of PD falls by the M1 PAM TAK-071

RATIONALE

In addition to the disease-defining motor symptoms, patients with Parkinson's disease (PD) exhibit gait dysfunction, postural instability, and a propensity for falls. These dopamine (DA) replacement-resistant symptoms in part have been attributed to loss of basal forebrain (BF) cholinergic neurons and, in interaction with striatal dopamine (DA) loss, to the resulting disruption of the attentional control of balance and complex movements. Rats with dual cholinergic-DA losses ("DL rats") were previously demonstrated to model PD falls and associated impairments of gait and balance.

OBJECTIVES

We previously found that the muscarinic M1-positive allosteric modulator (PAM) TAK-071 improved the attentional performance of rats with BF cholinergic losses. Here, we tested the hypotheses that TAK-071 reduces fall rates in DL rats.

RESULTS

Prior to DL surgery, female rats were trained to traverse a rotating straight rod as well as a rod with two zigzag segments. DL rats were refamiliarized with such traversals post-surgery and tested over 7 days on increasingly demanding testing conditions. TAK-071 (0.1, 0.3 mg/kg, p.o.) was administered prior to daily test sessions over this 7-day period. As before, DL rats fell more frequently than sham-operated control rats. Treatment of DL rats with TAK-071 reduced falls from the rotating rod and the rotating zigzag rod, specifically when the angled part of the zigzag segment, upon entering, was at a steep, near vertical angle.

CONCLUSIONS

TAK-071 may benefit complex movement control, specifically in situations which disrupt the patterning of forward movement and require the interplay between cognitive and motor functions to modify movement based on information about the state of dynamic surfaces, balance, and gait.

Social Symptoms of Parkinson's Disease

Parkinson's disease (PD) is typically well recognized by its characteristic motor symptoms (e.g., bradykinesia, rigidity, and tremor). The cognitive symptoms of PD are increasingly being acknowledged by clinicians and researchers alike. However, PD also involves a host of emotional and communicative changes which can cause major disruptions to social functioning. These incude problems producing emotional facial expressions (i.e., facial masking) and emotional speech (i.e., dysarthria), as well as difficulties recognizing the verbal and nonverbal emotional cues of others. These social symptoms of PD can result in severe negative social consequences, including stigma, dehumanization, and loneliness, which might affect quality of life to an even greater extent than more well-recognized motor or cognitive symptoms. It is, therefore, imperative that researchers and clinicans become aware of these potential social symptoms and their negative effects, in order to properly investigate and manage the socioemotional aspects of PD. This narrative review provides an examination of the current research surrounding some of the most common social symptoms of PD and their related social consequences and argues that proactively and adequately addressing these issues might improve disease outcomes.

Cognitive impairment in Parkinson's disease: A clinical and pathophysiological overview

Cognitive dysfunction in Parkinson's disease (PD) has received increasing attention, and, together with other non-motor symptoms, exert a significant functional impact in the daily lives of patients. This article aims to compile and briefly summarize selected published data about clinical features, cognitive evaluation, biomarkers, and pathophysiology of PD-related dementia (PDD). The literature search included articles indexed in the MEDLINE/PubMed database, published in English, over the last two decades. Despite significant progress on clinical criteria and cohort studies for PD-mild cognitive impairment (PD-MCI) and PDD, there are still knowledge gaps about its exact molecular and pathological basis. Here we overview the scientific literature on the role of functional circuits, neurotransmitter systems (monoaminergic and cholinergic), basal forebrain, and brainstem nuclei dysfunction in PD-MCI. Correlations between neuroimaging and cerebrospinal fluid (CSF) biomarkers, clinical outcomes, and pathological results are described to aid in uncovering the neurodegeneration pattern in PD-MCI and PDD.

Cognitive Rehabilitation for Individuals With Parkinson's Disease: Developing and Piloting an External Aids Treatment Program

Purpose Cognitive deficits are common in Parkinson's disease (PD) and can have a detrimental effect on daily activities. To date, most cognitive treatments have had an impairment-based focus with primary outcome measures of formal neuropsychological test scores. Few, if any, studies have focused on functional improvement or patient-centered goals. Method Three individuals with idiopathic PD participated in an 8-week pilot treatment program to train for the use of compensatory external aids to achieve personalized goals. Goal attainment scaling was the primary outcome measure, which was independently judged by multiple raters at baseline, postintervention, and 1 month posttreatment and analyzed via T-score analysis. Descriptive measures, including self-report and spouse-report rating scales of cognitive functioning, were employed. Results All 3 participants improved in the majority of their laboratory and home goals posttreatment, as measured by goal attainment scaling, and maintained gains for the majority of goals 1 month posttreatment. Conclusions This is the 1st known study to implement an external aids treatment program with patient-centered goals for individuals with cognitive deficits from PD. Positive outcomes were likely influenced by 3 factors: (a) a theoretically motivated focus on external aids; (b) a well-documented, systematic approach to instruction; and (c) the personalization of goals. Supplemental Material https://doi.org/10.23641/asha.10093493.

Visual Discrimination, Serial Reversal, and Extinction Learning in the mdx Mouse

Duchenne muscular dystrophy (DMD) is the most common form of muscular dystrophy and the most common neuromuscular disorder. In addition to neuromuscular consequences, some individuals with DMD experience global intellectual dysfunction and executive dysfunction of unknown mechanistic origin. The cognitive profile of the mdx mouse, the most commonly used mouse model of DMD, has been incompletely characterized and has never been assessed using the touchscreen operant conditioning paradigm. The touchscreen paradigm allows the use of protocols that are virtually identical to those used in human cognitive testing and may, therefore, provide the most translational paradigm for quantifying mouse cognitive function. In the present study, we used the touchscreen paradigm to assess the effects of the mdx mutation on visual discrimination learning, serial reversal learning, and extinction learning. To enable measuring task-dependent learning and memory processes while holding demands on sensory-driven information processing constant, we developed equally salient visual stimuli and used them on all experimental stages. Acquisition of the initial pairwise visual discrimination was facilitated in mdx mice relative to wildtype littermates; this effect was not explained by genotypic differences in impulsivity, motivation, or motor deficits. The mdx mutation had no effect on serial reversal or extinction learning. Together, findings from this study and previous studies suggest that mdx effects on cognitive function are task-specific and may be influenced by discrimination type (spatial, visual), reward type (food, escape from a non-preferred environment), sex, and genetic background.

Executive Functioning, Muscle Power and Reactive Balance Are Major Contributors to Gait Adaptability in People With Parkinson's Disease

Background and Aim: The ability to adapt gait when negotiating unexpected hazards is crucial to maintain stability and avoid falling. This study investigated cognitive, physical and psychological factors associated with gait adaptability required for obstacle and stepping target negotiation in people with Parkinson’s disease (PD).

Methods: Fifty-four people with PD were instructed to either: (a) avoid an obstacle at usual step distance; or (b) step onto a target at either a short or long step distance projected on a walkway two heel strikes ahead and then continue walking. Participants also completed clinical [Hoehn & Yahr rating scale; Movement Disorders Society version of the Unified Parkinson’s Disease Rating Scale motor section (MDS-UPDRS-III)], cognitive [simple reaction time, Trail Making and Stroop stepping (difference between incongruent and standard Choice Stepping Reaction Time, CSRT) tests], physical [hip abductor muscle power and reactive balance (pull test from the MDS-UPDRS-III)] and psychological (Fall Efficacy Scale–International) assessments.

Results: Discriminant function analysis revealed Stroop stepping test (inhibitory control) performance was the best predictor of stepping errors across the Gait Adaptability Test (GAT) conditions. Poorer executive function [Trail Making Test (TMT)] and reactive balance predicted poorer stepping accuracy in the short target condition; poorer reactive balance predicted increased number of steps taken to approach the obstacle and the long target; and poorer executive function predicted obstacle avoidance. Weaker hip abductor muscle power, poorer reactive balance, slower reaction time, poorer executive function and higher concern about falling were significant predictors of shorter step length while negotiating the obstacle/targets.

Conclusion: Superior executive function, effective reactive balance and good muscle power were associated with successful gait adaptability. Executive function and reactive balance appear particularly important for precise foot placements; and cognitive capacity for step length adjustments for avoiding obstacles. These findings suggest that impaired inhibitory control contributes to stepping errors and may increase fall risk in people with PD. These findings help elucidate mechanisms for why people with PD fall and may facilitate fall risk assessments and fall prevention strategies for this group.

Problems with Facial Mimicry Might Contribute to Emotion Recognition Impairment in Parkinson's Disease

Difficulty with emotion recognition is increasingly being recognized as a symptom of Parkinson's disease. Most research into this area contends that progressive cognitive decline accompanying the disease is to be blamed. However, facial mimicry (i.e., the involuntary congruent activation of facial expression muscles upon viewing a particular facial expression) might also play a role and has been relatively understudied in this clinical population. In healthy participants, facial mimicry has been shown to improve recognition of observed emotions, a phenomenon described by embodied simulation theory. Due to motor disturbances, Parkinson's disease patients frequently show reduced emotional expressiveness, which translates into reduced mimicry. Therefore, it is likely that facial mimicry problems in Parkinson's disease contribute at least partly to the emotional recognition deficits that these patients experience and might greatly influence their social cognition abilities and quality of life. The present review aims to highlight the need for further inquiry into the motor mechanisms behind emotional recognition in Parkinson's disease by synthesizing behavioural, physiological, and neuroanatomical evidence.

The Functional Alterations in Top-Down Attention Streams of Parkinson's disease Measured by EEG

Early and moderate Parkinson’s disease patients seem to have attention dysfunctions manifested differentially in separate attention streams: top-down and bottom-up. With a focus on the neurophysiological underpinnings of such differences, this study evaluated source-localized regional activity and functional connectivity of regions in the top-down and bottom-up streams as well as any discordance between the two streams. Resting state electroencephalography was used for 36 Parkinson’s disease patients and 36 healthy controls matched for age and gender. Parkinson’s disease patients showed disproportionally higher bilateral gamma activity in the bottom-up stream and higher left alpha2 connectivity in the top-down stream when compared to age-matched controls. An additional cross-frequency coupling analysis showed that Parkinson’s patients have higher alpha2-gamma coupling in the right posterior parietal cortex, which is part of the top-down stream. Higher coupling in this region was also associated with lower severity of motor symptoms in Parkinson’s disease. This study provides evidence that in Parkinson’s disease, the activity in gamma frequency band and connectivity in alpha2 frequency band is discordant between top-down and bottom-up attention streams.

Standardised Neuropsychological Assessment for the Selection of Patients Undergoing DBS for Parkinson's Disease

DBS is an increasingly offered advanced treatment for Parkinson's disease (PD). Neuropsychological assessment is considered to be an important part of the screening for selection of candidates for this treatment. However, no standardised screening procedure currently exists. In this study, we examined the use of our standardised neuropsychological assessment for the evaluation of surgical candidates and to identify risk factors for subsequent decline in cognition and mood. A total of 40 patients were assessed before and after DBS. Evaluation of mood and case notes review was also undertaken. Before DBS, patients with PD demonstrated frequent impairments in intellectual functioning, memory, attention, and executive function, as well as high rates of mood disorder. Post-DBS, there was a general decline in verbal fluency only, and in one patient, we documented an immediate and irreversible global cognitive decline, which was associated with older age and more encompassing cognitive deficits at baseline. Case note review revealed that a high proportion of patients developed mood disorder, which was associated with higher levels of depression at baseline and greater reduction in levodopa medication. We conclude that our neuropsychological assessment is suitable for the screening of candidates and can identify baseline risk factors, which requires careful consideration before and after surgery.

Impaired topological architecture of brain structural networks in idiopathic Parkinson's disease: a DTI study

Parkinson's disease (PD) is considered as a neurodegenerative disorder of the brain central nervous system. But, to date, few studies adopted the network model to reveal topological changes in brain structural networks in PD patients. Additionally, although the concept of rich club organization has been widely used to study brain networks in various brain disorders, there is no study to report the changed rich club organization of brain networks in PD patients. Thus, we collected diffusion tensor imaging (DTI) data from 35 PD patients and 26 healthy controls and adopted deterministic tractography to construct brain structural networks. During the network analysis, we calculated their topological properties, and built the rich club organization of brain structural networks for both subject groups. By comparing the between-group differences in topological properties and rich club organizations, we found that the connectivity strength of the feeder and local connections are lower in PD patients compared to those of the healthy controls. Furthermore, using a network-based statistic (NBS) approach, we identified uniformly significantly decreased connections in two modules, the limbic/paralimbic/subcortical module and the cognitive control/attention module, in patients compared to controls. In addition, for the topological properties of brain network topology in the PD patients, we found statistically increased shortest path length and decreased global efficiency. Statistical comparisons of nodal properties were also widespread in the frontal and parietal regions for the PD patients. These findings may provide useful information to better understand the abnormalities of brain structural networks in PD patients.

Dopaminergic Therapy Increases Go Timeouts in the Go/No-Go Task in Patients with Parkinson's Disease

Parkinson's disease (PD) is characterized by resting tremor, rigidity and bradykinesia. Dopaminergic medications such as L-dopa treat these motor symptoms, but can have complex effects on cognition. Impulse control is an essential cognitive function. Impulsivity is multifaceted in nature. Motor impulsivity involves the inability to withhold pre-potent, automatic, erroneous responses. In contrast, cognitive impulsivity refers to improper risk-reward assessment guiding behavior. Informed by our previous research, we anticipated that dopaminergic therapy would decrease motor impulsivity though it is well known to enhance cognitive impulsivity. We employed the Go/No-go paradigm to assess motor impulsivity in PD. Patients with PD were tested using a Go/No-go task on and off their normal dopaminergic medication. Participants completed cognitive, mood, and physiological measures. PD patients on medication had a significantly higher proportion of Go trial Timeouts (i.e., trials in which Go responses were not completed prior to a deadline of 750 ms) compared to off medication (p = 0.01). No significant ON-OFF differences were found for Go trial or No-go trial response times (RTs), or for number of No-go errors. We interpret that dopaminergic therapy induces a more conservative response set, reflected in Go trial Timeouts in PD patients. In this way, dopaminergic therapy decreased motor impulsivity in PD patients. This is in contrast to the widely recognized effects of dopaminergic therapy on cognitive impulsivity leading in some patients to impulse control disorders. Understanding the nuanced effects of dopaminergic treatment in PD on cognitive functions such as impulse control will clarify therapeutic decisions.

Stepping reaction time and gait adaptability are significantly impaired in people with Parkinson's disease: Implications for fall risk

BACKGROUND

Decline in the ability to take effective steps and to adapt gait, particularly under challenging conditions, may be important reasons why people with Parkinson's disease (PD) have an increased risk of falling. This study aimed to determine the extent of stepping and gait adaptability impairments in PD individuals as well as their associations with PD symptoms, cognitive function and previous falls.

METHODS

Thirty-three older people with PD and 33 controls were assessed in choice stepping reaction time, Stroop stepping and gait adaptability tests; measurements identified as fall risk factors in older adults.

RESULTS

People with PD had similar mean choice stepping reaction times to healthy controls, but had significantly greater intra-individual variability. In the Stroop stepping test, the PD participants were more likely to make an error (48 vs 18%), took 715 ms longer to react (2312 vs 1517 ms) and had significantly greater response variability (536 vs 329 ms) than the healthy controls. People with PD also had more difficulties adapting their gait in response to targets (poorer stepping accuracy) and obstacles (increased number of steps) appearing at short notice on a walkway. Within the PD group, higher disease severity, reduced cognition and previous falls were associated with poorer stepping and gait adaptability performances.

CONCLUSIONS

People with PD have reduced ability to adapt gait to unexpected targets and obstacles and exhibit poorer stepping responses, particularly in a test condition involving conflict resolution. Such impaired stepping responses in Parkinson's disease are associated with disease severity, cognitive impairment and falls.

Automatic Online Motor Control Is Intact in Parkinson's Disease With and Without Perceptual Awareness

In the double-step paradigm, healthy human participants automatically correct reaching movements when targets are displaced. Motor deficits are prominent in Parkinson's disease (PD) patients. In the lone investigation of online motor correction in PD using the double-step task, a recent study found that PD patients performed unconscious adjustments appropriately but seemed impaired for consciously-perceived modifications. Conscious perception of target movement was achieved by linking displacement to movement onset. PD-related bradykinesia disproportionately prolonged preparatory phases for movements to original target locations for patients, potentially accounting for deficits. Eliminating this confound in a double-step task, we evaluated the effect of conscious awareness of trajectory change on online motor corrections in PD. On and off dopaminergic therapy, PD patients (n = 14) and healthy controls (n = 14) reached to peripheral visual targets that remained stationary or unexpectedly moved during an initial saccade. Saccade latencies in PD are comparable to controls'. Hence, target displacements occurred at equal times across groups. Target jump size affected conscious awareness, confirmed in an independent target displacement judgment task. Small jumps were subliminal, but large target displacements were consciously perceived. Contrary to the previous result, PD patients performed online motor corrections normally and automatically, irrespective of conscious perception. Patients evidenced equivalent movement durations for jump and stay trials, and trajectories for patients and controls were identical, irrespective of conscious perception. Dopaminergic therapy had no effect on performance. In summary, online motor control is intact in PD, unaffected by conscious perceptual awareness. The basal ganglia are not implicated in online corrective responses.

Dorsal striatum mediates deliberate decision making, not late-stage, stimulus-response learning

We investigated a controversy regarding the role of the dorsal striatum (DS) in deliberate decision-making versus late-stage, stimulus-response learning to the point of automatization. Participants learned to associate abstract images with right or left button presses explicitly before strengthening these associations through stimulus-response trials with (i.e., Session 1) and without (i.e., Session 2) feedback. In Session 1, trials were divided into response-selection and feedback events to separately assess decision versus learning processes. Session 3 evaluated stimulus-response automaticity using a location Stroop task. DS activity correlated with response-selection and not feedback events in Phase 1 (i.e., Blocks 1-3), Session 1. Longer response times (RTs), lower accuracy, and greater intertrial variability characterized Phase 1, suggesting deliberation. DS activity extinguished in Phase 2 (i.e., Blocks 4-12), Session 1, once RTs, response variability, and accuracy stabilized, though stimulus-response automatization continued. This was signaled by persisting improvements in RT and accuracy into Session 2. Distraction between Sessions 1 and 2 briefly reintroduced response uncertainty, and correspondingly, significant DS activity reappeared in Block 1 of Session 2 only. Once stimulus-response associations were again refamiliarized and deliberation unnecessary, DS activation disappeared for Blocks 2-8, Session 2. Interference from previously learned right or left button responses with incongruent location judgments in a location Stroop task provided evidence that automaticity of stimulus-specific button-press responses had developed by the end of Session 2. These results suggest that DS mediates decision making and not late-stage learning, reconciling two, independently evolving and well-supported literatures that implicate DS in different cognitive functions. Hum Brain Mapp 38:6133-6156, 2017. © 2017 Wiley Periodicals, Inc.

Freezing-related perception deficits of asymmetrical walking in Parkinson's disease

Patients with Parkinson's disease (PD), and especially those with freezing of gait (FOG), are known to experience impairments in gait rhythmicity, symmetry, and bilateral coordination between both legs. In the current study, we investigated whether deficits in perception of gait speed between limbs were more pronounced in freezers than in non-freezers and could explain some of these gait impairments. We also assessed cognitive ability and proprioception. Twenty-five PD patients (13 freezers, 12 non-freezers) and 12 healthy controls walked on a split-belt treadmill, while the speed of one of the belts was gradually increased. Participants had to indicate the moment at which they perceived belt speeds to be different. The main outcome variables were the number of correct responses (perception accuracy) and the difference in belt speeds at the moment the participants perceived belt speeds to be different (perception threshold). In addition, gait characteristics during both split- and tied-belt walking were determined. Results showed significantly lower perception accuracy in freezers, whereas the perception threshold did not differ between groups. During tied-belt walking, freezers exhibited more asymmetrical step lengths and limb excursions than non-freezers and healthy controls. Greater step length and limb excursions were associated with better perception, whereas more variable gait was associated with more impaired perception. The results confirm the hypothesis that freezers have impaired perception of locomotor asymmetry. While proprioceptive and cognitive ability did not explain these findings, the possible causal link with the occurrence of FOG needs further corroboration.

Spatial structure normalises working memory performance in Parkinson's disease

Cognitive deficits are a frequent symptom of Parkinson's disease (PD), particularly in the domain of spatial working memory (WM). Despite numerous demonstrations of aberrant WM in patients, there is a lack of understanding about how, if at all, their WM is fundamentally altered. Most notably, it is unclear whether span - the yardstick upon which most WM models are built - is compromised by the disease. Moreover, it is also unknown whether WM deficits occur in all patients or only exist in a sub-group who are executively impaired. We assessed the factors that influenced spatial span in medicated patients by varying the complexity of to-be-remembered items. Principally, we manipulated the ease with which items could enter - or be blocked from - WM by varying the level of structure in memoranda. Despite having similar levels of executive performance to controls, PD patients were only impaired when remembering information that lacked spatial, easy-to-chunk, structure. Patients' executive function, however, did not influence this effect. The ease with which patients could control WM was further examined by presenting irrelevant information during encoding, varying the level of structure in irrelevant information and manipulating the amount of switching between relevant and irrelevant information. Disease did not significantly alter the effect of these manipulations. Rather, patients' executive performance constrained the detrimental effect of irrelevant information on WM. Thus, PD patients' spatial span is predominantly determined by level of structure in to-be-remembered information, whereas their level of executive function may mitigate against the detrimental effect of irrelevant information.

Pramipexole Increases Go Timeouts but Not No-go Errors in Healthy Volunteers

Parkinson's disease (PD) is characterized by motor symptoms, such as resting tremor, bradykinesia and rigidity, but also features non-motor complications. PD patients taking dopaminergic therapy, such as levodopa but especially dopamine agonists (DAs), evidence an increase in impulse control disorders (ICDs), suggesting a link between dopaminergic therapy and impulsive pursuit of pleasurable activities. However, impulsivity is a multifaceted construct. Motor impulsivity refers to the inability to overcome automatic responses or cancel pre-potent responses. Previous research has suggested that PD patients, on dopaminergic medications, have decreased motor impulsivity. Whether effects on impulsivity are main effects of dopaminergic therapies or are specific to PD is unclear. Using a Go No-go task, we investigated the effect of a single dose of the DA pramipexole on motor impulsivity in healthy participants. The Go No-go task consisted of Go trials, for which keystroke responses were made as quickly as possible, and lesser frequency No-go trials, on which motor responses were to be inhibited. We hypothesized that pramipexole would decrease motor impulsivity. This would manifest as: (a) fewer No-go errors (i.e., fewer responses on trials in which a response ought to have been inhibited); and (b) more timed-out Go trials (i.e., more trials on which the deadline elapsed before a decision to make a keystroke occurred). Healthy volunteers were treated with either 0.5 mg of pramipexole or a standard placebo (randomly determined). During the 2-h wait period, they completed demographic, cognitive, physiological and affective measures. The pramipexole group had significantly more Go timeouts (p < 0.05) compared to the placebo group though they did not differ in percent of No-go errors. In contrast to its effect on pursuit of pleasurable activities, pramipexole did not increase motor impulsivity. In fact, in line with findings in PD and addiction, dopaminergic therapy might increase motor impulse control. In these patient groups, by enhancing function of the dorsal striatum (DS) of the basal ganglia in contrast to its effect on impulsive pursuit of pleasurable activities. These findings have implications for use and effects of pramipexole in PD as well as in other conditions (e.g., restless leg, dystonia, depression, addiction-related problems).

Action Experience and Action Discovery in Medicated Individuals with Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disorder that markedly affects voluntary action. While regular dopamine treatment can help restore motor function, dopamine also influences cognitive portions of the action system. Previous studies have demonstrated that dopamine medication boosts action-effect associations, which are crucial for the discovery of new voluntary actions. In the present study, we investigated whether neural processes involved in the discovery of new actions are altered in PD participants on regular dopamine treatment, compared to healthy age-matched controls. We recorded brain electroencephalography (EEG) activity while PD patients and age-matched controls performed action discovery (AD) and action control tasks. We found that the novelty P3, a component normally present when there is uncertainty about the occurrence of the sensory effect, was enhanced in PD patients. However, AD was maintained in PD patients, and the novelty P3 demonstrated normal learning-related reductions. Crucially, we found that in PD patients the causal association between an action and its resulting sensory outcome did not modulate the amplitude of the feedback correct-related positivity (fCRP), an EEG component sensitive to the association between an action and its resulting effect. Collectively, these preliminary results suggest that the formation of long-term action-outcome representations may be maintained in PD patients on regular dopamine treatment, but the initial experience of action-effect association may be affected.

An Operant Intra-/Extra-dimensional Set-shift Task for Mice

Alterations in executive control and cognitive flexibility, such as attentional set-shifting abilities, are core features of several neuropsychiatric diseases. The most widely used neuropsychological tests for the evaluation of attentional set-shifting in human subjects are the Wisconsin Card Sorting Test (WCST) and the CANTAB Intra-/Extra-dimensional set shift task (ID/ED). These tasks have proven clinical relevance and have been modified and successfully adapted for research in animal models. However, currently available tasks for rodents present several limitations, mainly due to their manual-based testing procedures, which are hampering translational advances in psychiatric medicine. To overcome these limitations and to better mimic the original version in primates, we present the development of a novel operant-based two-chamber ID/ED "Operon" task for rodents. We demonstrated the effectiveness of this novel task to measure different facets of cognitive flexibility in mice including attentional set formation and shifting, and reversal learning. Moreover, we show the high flexibility of this task in which three different perceptual dimensions can be manipulated with a high number of stimuli cues for each dimension. This novel ID/ED Operon task can be an effective preclinical tool for drug testing and/or large genetic screening relevant to the study of executive dysfunction and cognitive symptoms found in psychiatric disorders.

The basal ganglia select the expected sensory input used for predictive coding

While considerable evidence supports the notion that lower-level interpretation of incoming sensory information is guided by top-down sensory expectations, less is known about the source of the sensory expectations or the mechanisms by which they are spread. Predictive coding theory proposes that sensory expectations flow down from higher-level association areas to lower-level sensory cortex. A separate theory of the role of prediction in cognition describes "emulations" as linked representations of potential actions and their associated expected sensation that are hypothesized to play an important role in many aspects of cognition. The expected sensations in active emulations are proposed to be the top-down expectation used in predictive coding. Representations of the potential action and expected sensation in emulations are claimed to be instantiated in distributed cortical networks. Combining predictive coding with emulations thus provides a theoretical link between the top-down expectations that guide sensory expectations and the cortical networks representing potential actions. Now moving to theories of action selection, the basal ganglia has long been proposed to select between potential actions by reducing inhibition to the cortical network instantiating the desired action plan. Integration of these isolated theories leads to the novel hypothesis that reduction in inhibition from the basal ganglia selects not just action plans, but entire emulations, including the sensory input expected to result from the action. Basal ganglia disinhibition is hypothesized to both initiate an action and also allow propagation of the action's associated sensory expectation down towards primary sensory cortex. This is a novel proposal for the role of the basal ganglia in biasing perception by selecting the expected sensation, and initiating the top-down transmission of those expectations in predictive coding.

Different attentional abilities among inbred mice strains using virtual object recognition task (VORT): SNAP25⁺/⁻ mice as a model of attentional deficit

Autism spectrum disorder (ASD), attention-deficit hyperactivity disorder (ADHD), schizophrenia, Alzheimer's and Parkinson's disease are characterized by attentional deficits. In the present study we first applied the virtual object recognition test (VORT), where 3D objects were replaced with highly discriminated geometrical shapes and presented on two 3.5-inch widescreen displays, in different inbred mice strains (C57BL/6N, DBA/2J, BALB/cJ), in comparison with the standard object recognition test (NOR). In both NOR and VORT, there was a progressive decay of performance in terms of reduced discrimination index from 5 min to 72 h of inter-trial delay in all strains. However, BALB/cJ inbred mice showed a better long lasting performance than C57BL/6N and DBA/2J, when tested in NOR. In VORT, BALB/cJ showed the best performance. Total exploration time was always higher in BALB/cJ than C57BL/6N and DBA/2J mice. C57BL/6N were less explorative strain than DBA/2J and BALB/cJ mice. When VORT was applied to SNAP-25(+/-) mice, an impairment in both NOR and VORT was shown. However, when moving shapes were applied, these heterozygous mice improved their performance, suggesting that the introduction of motion is a strong cue that makes the task more valuable to study attention deficits. Taken together, these data indicate that VORT provides a useful and rapid tool to identify the attentional deficit in different inbred strains and genetically modified mice, enhancing the value of psychiatric mouse models.

Impaired Early Attentional Processes in Parkinson's Disease: A High-Resolution Event-Related Potentials Study

Introduction

The selection of task-relevant information requires both the focalization of attention on the task and resistance to interference from irrelevant stimuli. A previous study using the P3 component of the event-related potentials suggested that a reduced ability to resist interference could be responsible for attention disorders at early stages of Parkinson’s disease (PD), with a possible role of the dorsolateral prefrontal cortex (DLPFC).

Methods

Our objective was to better determine the origin of this impairment, by studying an earlier ERP component, the N2, and its subcomponents, as they reflect early inhibition processes and as they are known to have sources in the anterior cingulate cortex (ACC), which is involved together with the DLPFC in inhibition processes. Fifteen early-stage PD patients and 15 healthy controls (HCs) performed a three-stimulus visual oddball paradigm, consisting in detecting target inputs amongst standard stimuli, while resisting interference from distracter ones. A 128-channel electroencephalogram was recorded during this task and the generators of the N2 subcomponents were identified using standardized weighted low-resolution electromagnetic tomography (swLORETA).

Results

PD patients displayed fewer N2 generators than HCs in both the DLPFC and the ACC, for all types of stimuli. In contrast to controls, PD patients did not show any differences between their generators for different N2 subcomponents.

Conclusion

Our data suggest that impaired inhibition in PD results from dysfunction of the DLPFC and the ACC during the early stages of attentional processes.

Perceptual, cognitive, and personality rigidity in Parkinson's disease

Parkinson's disease (PD) is associated with motor and non-motor rigidity symptoms (e.g., cognitive and personality). The question is raised as to whether rigidity in PD also extends to perception, and if so, whether perceptual, cognitive, and personality rigidities are correlated. Bistable stimuli were presented to 28 non-demented individuals with PD and 26 normal control adults (NC). Necker cube perception and binocular rivalry were examined during passive viewing, and the Necker cube was additionally used for two volitional-control conditions: Hold one percept in front, and Switch between the two percepts. Relative to passive viewing, PD were significantly less able than NC to reduce dominance durations in the Switch condition, indicating perceptual rigidity. Tests of cognitive flexibility and a personality questionnaire were administered to explore the association with perceptual rigidity. Cognitive flexibility was not correlated with perceptual rigidity for either group. Personality (novelty seeking) correlated with dominance durations on Necker passive viewing for PD but not NC. The results indicate the presence in mild-moderate PD of perceptual rigidity and suggest shared neural substrates with novelty seeking, but functional divergence from those supporting cognitive flexibility. The possibility is raised that perceptual rigidity may be a harbinger of cognitive inflexibility later in the disease course.

Effect of visual cues on the resolution of perceptual ambiguity in Parkinson's disease and normal aging

Parkinson's disease (PD) and normal aging have been associated with changes in visual perception, including reliance on external cues to guide behavior. This raises the question of the extent to which these groups use visual cues when disambiguating information. Twenty-seven individuals with PD, 23 normal control adults (NC), and 20 younger adults (YA) were presented a Necker cube in which one face was highlighted by thickening the lines defining the face. The hypothesis was that the visual cues would help PD and NC to exert better control over bistable perception. There were three conditions, including passive viewing and two volitional-control conditions (hold one percept in front; and switch: speed up the alternation between the two). In the Hold condition, the cue was either consistent or inconsistent with task instructions. Mean dominance durations (time spent on each percept) under passive viewing were comparable in PD and NC, and shorter in YA. PD and YA increased dominance durations in the Hold cue-consistent condition relative to NC, meaning that appropriate cues helped PD but not NC hold one perceptual interpretation. By contrast, in the Switch condition, NC and YA decreased dominance durations relative to PD, meaning that the use of cues helped NC but not PD in expediting the switch between percepts. Provision of low-level cues has effects on volitional control in PD that are different from in normal aging, and only under task-specific conditions does the use of such cues facilitate the resolution of perceptual ambiguity.

Disentangling the Role of Cortico-Basal Ganglia Loops in Top-Down and Bottom-Up Visual Attention: An Investigation of Attention Deficits in Parkinson Disease

It is solidly established that top-down (goal-driven) and bottom-up (stimulus-driven) attention mechanisms depend on distributed cortical networks, including prefrontal and frontoparietal regions. On the other hand, it is less clear whether the BG also contribute to one or the other of these mechanisms, or to both. The current study was principally undertaken to clarify this issue. Parkinson disease (PD), a neurodegenerative disorder primarily affecting the BG, has proven to be an effective model for investigating the contribution of the BG to different brain functions; therefore, we set out to investigate deficits of top-down and bottom-up attention in a selected cohort of PD patients. With this objective in mind, we compared the performance on three computerized tasks of two groups of 12 parkinsonian patients (assessed without any treatment), one otherwise pharmacologically treated and the other also surgically treated, with that of a group of controls. The main behavioral tool for our study was an attentional capture task, which enabled us to tap the competition between top-down and bottom-up mechanisms of visual attention. This task was suitably combined with a choice RT and a simple RT task to isolate any specific deficit of attention from deficits in motor response selection and initiation. In the two groups of patients, we found an equivalent increase of attentional capture but also comparable delays in target selection in the absence of any salient distractor (reflecting impaired top-down mechanisms) and movement initiation compared with controls. In contrast, motor response selection processes appeared to be prolonged only in the operated patients. Our results confirm that the BG are involved in both motor and cognitive domains. Specifically, damage to the BG, as it occurs in PD, leads to a distinct deficit of top-down control of visual attention, and this can account, albeit indirectly, for the enhancement of attentional capture, reflecting weakened ability of top-down mechanisms to antagonize bottom-up control.

Dopaminergic therapy affects learning and impulsivity in Parkinson's disease

OBJECTIVE

The aim was to examine the effect of dopaminergic medication on stimulus-response learning versus performing decisions based on learning.

METHOD

To see the effect of dopaminergic therapy on stimulus-response learning and response selection, participants with Parkinson's disease (PD) were either tested on and/or off their prescribed dose of dopaminergic therapy during different testing days. Forty participants with PD and 34 healthy controls completed the experiment on consecutive days. On Day 1, participants learned to associate abstract images with spoken, "right" or "left" responses via feedback (Session 1). On Day 2, participants recalled these responses (Session 2) and indicated the location (i.e., right or left of center) of previously studied images intermixed with new images (Session 3).

RESULTS

Participants with PD off medication learned stimulus-response associations equally well compared to healthy controls. Learning was impaired by dopaminergic medication. Regardless of medication status, patients recalled the stimulus-response associations from Day 1 as well as controls. In Session 3 off medication, patients demonstrated enhanced facilitation relative to controls and patients on medication, when the stimulus location was congruent with the spoken response that was learned for the stimulus in Session 1.

INTERPRETATION

Learning in PD was comparable to that of healthy controls off medication. Learning was worsened by dopaminergic therapy in PD. We interpret greater facilitation in participants with PD off medication for congruent responses as evidence of greater impulsivity. This motor or reflexive impulsivity was normalized by medication in PD. These findings shed light on the cognitive profile of PD and have implications for dopaminergic treatment.

Executive dysfunction in Parkinson's disease: a review

Executive dysfunction can be present from the early stages of Parkinson's disease (PD). It is characterized by deficits in internal control of attention, set shifting, planning, inhibitory control, dual task performance, and on a range of decision-making and social cognition tasks. Treatment with dopaminergic medication has variable effects on executive deficits, improving some, leaving some unchanged, and worsening others. In this review, we start by defining the specific nature of executive dysfunction in PD and describe suitable neuropsychological tests. We then discuss how executive deficits relate to pathology in specific territories of the basal ganglia, consider the impact of dopaminergic treatment on executive function (EF) in this context, and review the changes in EFs with disease progression. In later sections, we summarize correlates of executive dysfunction in PD with motor performance (e.g., postural instability, freezing of gait) and a variety of psychiatric (e.g., depression, apathy) and other clinical symptoms, and finally discuss the implications of these for the patients' daily life.

The contribution of acetylcholine and dopamine to subprocesses of visual working memory--what patients with amnestic mild cognitive impairment and Parkinson׳s disease can tell us

Attentional selection, i.e. filtering out of irrelevant sensory input and information storage are two crucial components of working memory (WM). It has been proposed that the two processes are mediated by different neurotransmitters, namely acetylcholine for attentional selection and dopamine for memory storage. However, this hypothesis has been challenged by others, who for example linked a lack in dopamine levels in the brain to filtering deficits. Here we tested the above mentioned hypothesis in two patient cohorts which either served as a proxy for a cholinergic or a dopaminergic deficit. The first group comprised 18 patients with amnestic mild cognitive impairment (aMCI), the second 22 patients with Parkinson׳s disease (PD). The two groups did not differ regarding their overall cognitive abilities. Both patient groups as well as a control group without neurological deficits (n=25) performed a visuo-spatial working memory task in which both the necessity to filter out irrelevant information and memory load, i.e. the number of items to be held in memory, were manipulated. In accordance with the primary hypothesis, aMCI patients displayed problems with filtering, i.e., were especially impaired when the task required ignoring distracting stimuli. PD patients on the other hand showed difficulties when memory load was increased suggesting that they mainly suffered from a storage deficit. In sum, this study underlines how the investigation of neurologic patients with a presumed neurotransmitter deficit can aid to clarify these neurotransmitters׳ contribution to specific cognitive functions.

Examining dorsal striatum in cognitive effort using Parkinson's disease and fMRI

OBJECTIVE

Understanding cognition mediated by the striatum can clarify cognitive deficits in Parkinson's disease (PD). Previously, we claimed that dorsal striatum (DS) mediates cognitive flexibility. To refute the possibility that variation in cognitive effort confounded our observations, we reexamined our data to dissociate cognitive flexibility from effort. PD provides a model for exploring DS-mediated functions. In PD, dopamine-producing cells supplying DS are significantly degenerated. DS-mediated functions are impaired off and improved on dopamine replacement medication. Functional magnetic resonance imaging (fMRI) can confirm striatum-mediated functions.

METHODS

Twenty-two PD patients, off-on dopaminergic medication, and 22 healthy age-matched controls performed a number selection task. Numerical distance between number pairs varied systematically. Selecting between two numbers that are closer versus distant in magnitude is more effortful: the symbolic distance effect. However, selecting between closer versus distant number pairs is equivalent in the need to alter attention or response strategies (i.e., cognitive flexibility). In Experiment 2, 28 healthy participants performed the same task with simultaneous measurement of brain activity with fMRI.

RESULTS

The symbolic distance effect was equivalent for PD versus control participants and across medication sessions. Furthermore, symbolic distance did not correlate with DS activation using fMRI. In this dataset, we showed previously that integrating conflicting influences on decision making is (1) impaired in PD and improved by dopaminergic therapy and (2) associated with preferential DS activation using fMRI.

INTERPRETATION

These findings support the notion that DS mediates cognitive flexibility specifically, not merely cognitive effort, accounting for some cognitive deficits in PD and informing treatment.

The ultimate intra-/extra-dimensional attentional set-shifting task for mice

BACKGROUND

Alterations in executive control and cognitive flexibility, such as attentional set-shifting abilities, are core features of several neuropsychiatric diseases. The most widely used neuropsychological tests for the evaluation of attentional set shifting in humans are the Wisconsin Card Sorting Test and the Cambridge Neuropsychological Test Automated Battery Intra-/Extra-Dimensional set-shift task (ID/ED). These tasks have proven clinical relevance and have been successfully adapted for monkeys. However, similar tasks currently available for rodents are limited, mainly because of their manual-based testing procedures. The current limitations of rodent attentional set-shifting tasks are hampering translational advances in psychiatric medicine.

METHODS

To closely mimic the Cambridge Neuropsychological Test Automated Battery ID/ED task in primates, we present the development of a novel operant-based two-chamber ID/ED "Operon" task for mice.

RESULTS

We show the ability of this novel task to measure attentional set shifting in mice and the effects of genetic and pharmacologic manipulations of dopamine and glutamate. In genetically modified mice with reduced catechol-O-methyltransferase activity there was selective improvement on extradimensional shift abilities and impairment of serial reversal learning. Chronic administration of phencyclidine produced a selective impairment of extradimensional shift while producing a generalized decrease in latency to respond.

CONCLUSIONS

We demonstrate that this novel ID/ED Operon task may be an effective preclinical tool for drug testing and large genetic screening relevant to the study of executive dysfunctions and cognitive symptoms of psychiatric disorders. These findings may help elucidate the biological validity of similar findings in humans.

Performance of C57BL/6J and DBA/2J mice on a touchscreen-based attentional set-shifting task

Attentional set-shifting deficits are a feature of multiple psychiatric disorders. However, the neurogenetic mechanisms underlying these deficits are largely unknown. In the present study we assessed performance of C57BL/6J and DBA/2J mice on a touchscreen-based attentional set-shifting task similar to those used with humans and non-human primates. In experiment 1, mice discriminated simple white lines followed by compound stimuli composed of white lines superimposed on grey shapes. Although performance of the two strains was largely equivalent during early stages of the task, DBA/2J mice committed significantly more errors compared to C57BL/6J mice on the extra-dimensional shift. Additionally, performance of mice as a group declined across the three compound discrimination reversals. In experiment 2 we assessed salience of the shapes and lines dimensions and determined if dimensional salience, a variable previously shown to affect set-shifting abilities in humans and non-human primates, could be systematically manipulated. Findings from experiment 2 suggested that strain differences during the extra-dimensional shift in experiment 1 were most parsimoniously explained by a consistently impaired ability in DBA/2J mice to discriminate a subset of the compound stimuli. Additionally, unlike maze-based tasks, the relative salience of the two dimensions could be manipulated by systematically altering the width of lines exemplars while retaining other potentially-relevant attributes of the compound stimuli. These findings reveal unique and in some cases strain-dependent phenomena related to discriminations of simple and multidimensional visual stimuli which may facilitate future efforts to identify and fully characterize visual discrimination, reversal learning, and attentional set-shifting deficits in mice.

Modeling fall propensity in Parkinson's disease: deficits in the attentional control of complex movements in rats with cortical-cholinergic and striatal-dopaminergic deafferentation

Cognitive symptoms, complex movement deficits, and increased propensity for falls are interrelated and levodopa-unresponsive symptoms in patients with Parkinson's disease (PD). We developed a test system for the assessment of fall propensity in rats and tested the hypothesis that interactions between loss of cortical cholinergic and striatal dopaminergic afferents increase fall propensity. Rats were trained to traverse stationary and rotating rods, placed horizontally or at inclines, and while exposed to distractors. Rats also performed an operant Sustained Attention Task (SAT). Partial cortical cholinergic and/or caudate dopaminergic deafferentation were produced by bilateral infusions of 192 IgG-saporin (SAP) into the basal forebrain and/or 6-hydroxydopamine (6-OHDA) into the caudate nucleus, respectively, modeling the lesions seen in early PD. Rats with dual cholinergic-dopaminergic lesions (DL) fell more frequently than SAP or 6-OHDA rats. Falls in DL rats were associated with incomplete rebalancing after slips and low traversal speed. Ladder rung walking and pasta handling performance did not indicate sensorimotor deficits. SAT performance was impaired in DL and SAP rats; however, SAT performance and falls were correlated only in DL rats. Furthermore, in DL rats, but not in rats with only dopaminergic lesions, the placement and size of dopaminergic lesion correlated significantly with fall rates. The results support the hypothesis that after dual cholinergic-dopaminergic lesions, attentional resources can no longer be recruited to compensate for diminished striatal control of complex movement, thereby "unmasking" impaired striatal control of complex movements and yielding falls.

Prospective memory in Parkinson disease during a virtual week: effects of both prospective and retrospective demands

OBJECTIVE

This study investigated the effect of Parkinson's disease (PD) on event-based prospective memory tasks with varying demand on (1) the amount of strategic attentional monitoring required for intention retrieval (prospective component), and (2) the retrospective memory processes required to remember the contents of the intention or the entire constellation of prospective memory tasks.

METHOD

Twenty-four older adults with PD and 28 healthy older adults performed the computerized Virtual Week task, a multi-intention prospective memory paradigm that simulates everyday prospective memory tasks. The Virtual Week included regular (low retrospective memory demand) and irregular (high retrospective memory demand) prospective memory tasks with cues that were focal (low strategic monitoring demand) or less focal (high strategic monitoring demand) to the ongoing activity.

RESULTS

For the regular prospective memory tasks, PD participants were impaired when the prospective memory cues were less focal. For the irregular prospective memory tasks, PD participants were impaired regardless of prospective memory cue type. PD participants also had impaired retrospective memory for irregular tasks, which was associated with worse prospective memory for these tasks during the Virtual Week.

CONCLUSIONS

When retrospective memory demands are minimized, prospective memory in PD can be supported by cues that reduce the executive control demands of intention retrieval. However, PD-related deficits in self-initiated encoding or planning processes have strong negative effects on the performance of prospective memory tasks, with increased retrospective memory demand.